1. Field of the Invention
The present invention relates to an optical disc reading apparatus for reading information recorded on an optical disc, and more particularly to an optical disc reproducing apparatus capable of correcting a tracking error caused by the lateral vibration (hereinafter, “surface wobbling”) of an optical disc or the vibration of a spindle motor.
2. Description of Related Art
Recently, optical discs such as a compact disc (CD) and a digital versatile disc (DVD) have been widely spread and popularly used. As an apparatus for reading information from and reproducing information recorded on the optical disc, for example, audio information or video information, there is provided an optical disc reproducing apparatus. Widely known amongst which are a CD player, a DVD player, and a CD-ROM drive connectable to a PC.
In an optical reproducing apparatus, there is provided an optical pickup device for reading information by irradiating an optical disc with a light beam. The optical pickup device emits a light beam to an information recorded surface of an optical disc that is placed on a turn table and rotating. Then, the light reflected by the optical disc is received by a light-receiving device, for example, a photo diode arranged in the optical pickup device. Thereafter, the light is converted into an electric signal by the light-receiving device so that the information recorded on the optical disc is outputted based on the obtained electric signal.
It is necessary to perform a process (tracking process) by aligning the center axis of the light beam with the center of a series of pits formed on the optical disc so as to read the information from the optical disc accurately. To achieve this, there are provided in the optical pickup device an actuator for driving an objective lens in the radial direction of the optical disc and a tracking servo for controlling the actuator.
Due to various factors, surface wobbling is sometimes caused in the optical disc. There are also cases in which, when the optical disc is placed on the turn table, peripheral edges thereof sag by its own weight, thereby preventing the information recorded surface of the disc from rotating in one plane. When such warpage or sag is minimal, it is possible to maintain a perpendicular relationship between the axis of the light beam and the information recorded surface of tracks. Therefore, the aforementioned tracking process by the tracking servo can be easily performed.
However, when the surface wobbling, the warpage, or the sag is conspicuous, there are some cases in which the relationship between the axis of the light beam and the information recorded surface is largely deviated from the perpendicular relationship. This causes the series of pits on the disc to be read to deviate largely in a radial direction from the axis of the light beam (tracking offset), generating a problem in which the information can not be read.
To deal with the aforementioned problem, JP-A-2000-163765 discloses an optical disc apparatus that performs a tracking control by correcting the tracking offset that is caused in accordance with the shift of the objective lens. The optical reproducing disc apparatus disclosed in JP-A-2000-163765 retains maximum and minimum values of the low frequency components of the phase difference signal obtained by a light-receiving device that is placed in a divided manner and generates a lens-shift detection signal from an average value of the maximum and minimum values. Then, the lens-shift detection signal thus generated is used for correcting the tracking offset.
Also, to deal with the aforementioned problem, JP-A-2000-315327 discloses an optical disc apparatus that performs a highly reliable control when the series of pits on a disc deviates from the optical axis of the light beam after the objective lens is shifted by the tracking servo. When the center of the objective lens is deviated due to the shift of the objective lens at an initial stage because of fitting errors of the optical components or vertical placement of the apparatus, the optical disc apparatus disclosed in JP-A-2000-315327 detects a signal corresponding to the amount of the deviation and makes a correction so that the amount of the deviation will be zero, i.e., so that the objective lens is located at an appropriate position.
The tracking control methods disclosed in JP-A-2000-163765 and JP-A-2000-315327 are complicated with respect to the processing procedures, which will require processing circuits to be newly added and disadvantageously make the construction of the entire apparatus complicated.